This invention relates to valve positioners for positioning fluid valves which control a process variable, and particularly LO a method for detecting malfunctioning components of a control valve, such as a damaged or broken valve stem, or a damaged plug or seat.
Valve positioners are in common usage for controlling fluid valves and associated fluids in oil and gas pipelines, chemical processing plants, etc. Typically the valve positioners include microprocessors for use in precisely controlling the process fluid and for performing diagnostic operations on the valve.
Various types of positioners are used in the process control industry. Some positioners are mechanically coupled to an actuator while some incorporate the actuator within the positioner. The actuator provides means for physically positioning the valve and may be either electrically, hydraulically or pneumatically operated. Electric actuators have a current signal which drives a motor which positions the valve. Hydraulic actuators have oil-filled means for positioning the valve. By far the most common type of valve actuator in the process control industryxe2x80x94a pneumatic actuator has a piston or a combination of a piston and a diaphragm. Depending on the application and the level of control integration, positioners receive several types of input from a controller which are representative of the desired valve position. One type is a current input having a 4-20 mA or 10-50 mA magnitude, a second is a digital signal superimposed on the current signal and a third is a fully digital input such as Fieldbus or HART(copyright). Alternatively, the positioner may receive a 3-15 pound per square inch (PSI) pneumatic input representative of the desired valve position. Depending on the level of integration and the application as well, positioners have different types of outputs. Some positioners provide an output current to a motor, while still others have a fast responding hydraulic output. The most common type of positioner output is a 0-200 PSI pneumatic pressure output. Positioners, as the word is used in this application includes all these field mounted instruments, including the various inputs and outputs, and their respective means for positioning valves if applicable.
In the most common case of a spring and piston/diaphragm actuator, the diaphragm deflects with the pressure delivered by the positioner, thereby exerting a force or torque on a control valve stem or rotary member, respectively, so as to change the position of the valve. Almost all positioners have a mechanical or an electronic position sensor to provide a position signal which is fed back into a microprocessor based control section of the positioner. No matter what the specific means are for delivering force to position a valve, positioners having microprocessor based control algorithms are known.
In U.S. Pat. Nos. 5,549,137; 5,558,115; and 5,573,032, all assigned to the same assignee as herein, there is provided a microprocessor based valve positioner which has real-time diagnostics to provide valve and actuator integrity information. All such microprocessor based valve positioners cannot easily detect a broken valve stem, a broken or damaged valve plug, or a broken or damaged valve seat. If the valve stem breaks for instance in such a fashion that the linkage to the positioner instrument is intact, the instrument will monitor feedback and position the actuator to what it thinks is the correct valve position without actually moving the valve plug. In a modern microprocessor based positioner instrument, the positioner should respond rapidly through a valve stem breakage event (typically in less than 100 ms). A fast instrument response would mask any possible host monitoring of valve performance, making the occurrence of the valve stem breakage continue undetected until some process change occurred due to the failed valve performance. In such conditions, the valve plug could fail in a manner that doesn""t greatly disturb the process, so the broken or damaged valve stem/plug/seat may continue undetected over a long period of time. Eventually this would lead to a loss of process control.
Actual detection of such broken valve components as a broken or damaged stem, plug, or seat, would require a vibration sensor or some other sophisticated mechanical sensor which would be expensive and also require add-on hardware to the positioner and valve assembly. It is therefore desirable to provide a technique which can be used to provide a sufficient indication of possible valve component breakage so as to suggest inspection is required by a technician. In particular, it would be desirable to provide such a stem loss/plug loss/seat loss detection technique which does not require additional mechanical components and which would be independent of any particular valve configuration, and thereby readily implemented to provide an early warning indication that there may be a breakage of a valve component.
In accordance with the principles of the present invention there is provided a method for indicating a possible failure in the valve stem or valve plug or valve seat in a configuration where a microprocessor based valve positioner is controlling a valve actuator and fluid control valve in a process control system.
In particular, the drive signal to the current to pressure (I/P) microprocessor based valve positioner is monitored; the drive pressure output of the valve positioner coupled to the valve actuator is also monitored for detecting a predetermined change from a steady drive pressure with no change in the set point drive signal input to the valve positioner. The valve stem position feedback is also monitored for detecting a predetermined change from a steady valve stem position feedback with no change in the set point drive signal. If at least one of the predetermined changes in steady drive pressure and the valve stem position feedback is detected, a stem/plug/seat event indication is provided.
Upon the occurrence of a stem/plug/seat event indication, there is initiated a monitoring of a process value for detecting any deviation in the process value, such as monitoring the relationship of the process flow rate for a set point value and detecting any changes in the relationship with no change in the set point. A deviation in the process value with no change in the set point indicates a possible valve stem breakage or a damaged valve plug or valve seat.
In implementing the method of the present invention, two alarm detections are run in parallel. The first alarm is for a valve disturbance alarm. Such an alarm would indicate a significant change, such as a greater than 10% change in drive pressure and/or valve position feedback without a corresponding change in set point. Detection of such a significant predetermined change would then enable detection of a second alarm.
The second alarm would look for a change in the set point to process value relationship with no change in the set point. If a difference was detected, then an event could be sent indicating a possible stem/plug/seat problem. The operator would then look for a process problem, which, if detected, would lead to a human inspection of the valve.
The significant advantage of the present invention is being able to detect problems with a valve at an early stage and thereby save the possible loss of expensive process fluid. Also, the present invention can be used to identify and select fluid control valves which may require inspection.